True owner

Only owner or agent of owner that has been vetsed with powers to sell say a POA, can sell. 

Agreeing with Mr. Bhartesh goyal; The POA to sell in case of immovable property should be registered.

In previous post I forgot to mention Registered.

The sale by third person who is not the owner of the property is not valid and is illegal in law.

No. Your perception is wrong. Lawyers only guide and lawyers are most genuine section of society. So pl correct your perception.

If such 3rd party had no connection to the property and is absolutely an isolated individual who has sold the property to someone, then the transfer is illegal and is voidable at the option of the true owner of the Property.

However, if such 3rd party has any apparent authority to sell the property to another person, then the concept of Ostensible Transfer of Property u/s 41 of Transfer of Property Act would apply, protecting the interests of such transferee in good faith (having taken reasonable care on his/her part). The burden of proof would here lie on the transferee to show the measures of reasonable care taken or to show that the 3rd party transferor ensured that the true authority of transfering the property remained out of sight of such transferee.

However, if no such ostensible authority was present there, the sale would probably fall under the Fraudulent Transfer and shall be voidable at the option of the true owner.